This invention relates to a process for obtaining useful products from chrome leather waste and to the products obtained thereby. More particularly, the invention relates to a process for converting chrome leather waste into protein hydrolysate and for simultaneously recovering chromium compounds.
As classified by source, there are two major categories of chrome leather scrap: one consists of pieces that are trimmed from chrome tanned hides and skins or fragmented when the chrome tanned hide is split into one or more layers; the other consists of shavings, dust or fiber bundles produced when the chrome tanned leather is machine shaved to a predetermined uniform thickness.
Chrome leather scrap, whether as shavings or pieces, may vary in moisture content from 60 percent down to 5 or ten percent depending upon the time it is stored after production and upon the conditions of storage. When completely dry, chrome leather scrap (unfinished by dyeing, fat liquoring, coloring or surface coating) contains approximately 3 to 4 percent chromium oxide (Cr.sub.2 O.sub.3 equivalent); 12 to 15 percent other inorganic salts; 3 to 5 percent fats, oil, and chloroform solubles; and seventy-five to eighty percent protein (hide collagen). Although one might encounter some inconsistency in such analytical figures, it should be remembered that many salts normally present in chrome leather (e.g., ammonium and amine salts, carbonates, formates, acetates, and other salts of organic acids) decompose when ashed.
At the present time there is an accumulation of this chrome leather scrap amounting to about one million pounds per day in the United States alone. Although there are several known uses for this material, there is little present interest in these uses, and the material is therefore of little value.
It has been known for may years that the fibrous nature of chrome leather waste makes it suitable for reconstruction into leather-like sheet or board. Such products, however, have enjoyed little commercial use.
Attempts have also been made to utilize chrome leather waste as a slow acting fertilizer, but the value of chrome leather scrape as fertilizer is so low that it has not been possible to justify the payment of freight to convey it to a central point for processing. Further although the chromium remains insoluble and cannot leach into ground water levels, the use of chrome leather waste as a fertilizer has been criticized on the basis that the chromium level might build up to a toxic level. While there has never been any substantiation for such criticism, the fear that it may have some basis tends to further reduce the value of chrome leather waste as a fertilizer.
The Department of Agriculture has sanctioned the use of one percent leather meal in swine feed, even though one effect is to add the equivalent of about 200 ppm of chromium metal to the feed ration. Still, there is real doubt as the value of chrome leather as a feed additive. This doubt is based more on the questionable digestability of leather scrap than it is on toxicity.
Prior to the advent of resins and other synthetic polymer adhesives, animal glue was a major adhesive material and was much in demand. This led to an outlet for chrome leather waste because it could be processed so as to yield animal glue or technical gelatin. This process involved prolonged soaking in lime or magnesia suspension to fix the chromium as insoluble chromites, washing out the excess alkali with cold water and finally extracting the glue or gelatin by successive warm and hot water cooks. Obviously, very mild conditions were preferred because the value of the extracted protein was inversely proportional to the degree of hydrolysis encountered. Therefore, the treatment with lime or magnesia or both to fix the chromium was always carried out at low temperatures for prolonged periods of time totalling several weeks. Secondly, all possible excess alkalinity was neutralized and washed out prior to hot water extraction so as to prevent hydrolysis of the protein.
Because the foregoing uses are not currently of substantial commercial interest, chrome leather scrap has become a drug on the market, and it has a very low value. Indeed, we now find that millions of pounds of chrome leather pieces and shavings are disposed of by dumping into sanitary land fills. It has not even proved feasible to use this scrap for the recovery of its two to three percent chromium oxide content.
Accordingly, a principal object of this invention is to provide an economically valuable use for chrome leather scrap, and more particularly, to provide a process for converting substantially all of the protein in chrome leather scrap into a nutritious protein hydrolysate.
A related object is to provide such a protein hydrolysate that is substantially free of heavy metal contaminants.
Still another object is to provide an economical process for recovering the insoluble chromium compounds from chrome leather scrap.
Additional objects, if not specifically described herein, will be readily apparent to those skilled in the arts of processing hide material into leather, glue, gelatin, and related rendered products.